Articles of cookware, such as pans of various sizes, may be made of a variety of substrate materials, such as ceramic, glass, or metal materials, such as stainless steel, copper, or aluminum. Often it is desirable to apply a coating to at least the interior surface of such cookware articles. Various types of coatings may be applied, including coatings designed to protect the inside surface of the cookware from damage due to abrasion and/or corrosion, and/or to provide a non-stick release coating on the inside surface of the cookware article so that food cooked in the cookware is easily removed therefrom.
Although various methods, such as roller and dip coating, may be used to apply a coating to the inside surface of an article of cookware, coatings are typically applied to cookware by spraying a desired coating material onto the interior surface of the cookware. Modern cookware coatings, such as non-stick release coatings, require several coating material application and curing steps to achieve the desired surface finish. For example, a non-stick release coating may be applied to the interior surface of a metal pan by applying a primer material to the inside pan surface, drying the primer layer in a hot air environment, applying one or more intermediate and top coats of non-stick material over the primer layer, and then curing or baking the pan once again in a hot air environment.
Effective and efficient processes have been developed for applying coatings to articles of cookware in a mass production industrial setting. Such industrial processes involve conveying a series of articles of cookware to be coated through each step in a coating process. A typical process for applying a coating to the inside surface of an article of cookware employs a chain-on-edge spray line. A chain-on-edge spray line employs a conveyor chain to transport articles of cookware through various stages in a process whereby coating materials are applied to the cookware and where the applied coating materials are dried or cured. The conveyor chain is formed of a plurality of links. Each link has a central aperture formed therein. A plurality of pan holders are mounted spaced apart on the conveyor chain. Each pan holder includes a pan holder chuck, whereby the pan holder is mounted on the conveyor chain, and a pan holding means, attached to the chuck, for holding a pan or other article which is to be transported through the coating process stages. The pan holder chuck includes a shaft, which extends downward through the central aperture in a link in the conveyor chain, and a pulley mounted to the shaft. The pulley is adapted to be engaged by a drive belt, which rotates the shaft, and, therefore, the entire pan holder, with respect to the conveyor chain. The pan holding means is typically implemented as a cylinder or other receptacle, having an upward facing opening formed therein adapted to receive a pan or other article of cookware to be coated. A pan blank, or other article of cookware to be coated, is dropped into the opening in the pan holding means. The pan is held in place in the holding means by the weight of the pan.
A pan held in the pan holder is transported through the various coating process stations by the conveyor chain. At a first processing station, for example, a primer material may be applied onto the interior surface of the pan. This may be achieved by extending a spray nozzle near or into the interior of the pan, and spraying a primer material in liquid form from the nozzle onto the interior surface of the pan. To ensure a relatively even coating of material is applied on the pan surface, the pan is preferably rotated as the primer material is sprayed thereon. A drive belt is thus provided in the primer material application station. The drive belt is positioned such that the pulley on the pan holder chuck engages the drive belt as the conveyor chain transports the pan holder through the primer application station. The drive belt rotates the pan holder, and the pan attached thereto, via the pulley while the primer coating material is sprayed onto the interior surface of the pan. Following the primer application station, the pan may be conveyed by the conveyor chain through a heat curing station, and one or more intermediate and/or top coat application stations, wherein, for example, one or more coats of non-stick coating material may be applied to the interior surface of the pan. Since the pan blank need not be rotated during the curing stage, a drive belt is typically not provided in the curing station. The intermediate and top coats of material may be applied in a manner similar to the manner in which the primer coating is applied, e.g., by spraying the material onto the interior surface of the pan while the pan is rotated. Thus, drive belts are typically provided in all of the coating material application stations to engage the pulley mounted on the pan holder chuck, to thereby rotate the pan holder and the pan held therein during the coating application process. After all coating material layers have been applied, the coated pan blank or other article is removed from the pan holder for further curing and/or processing steps.
For most conventional cookware coatings, such as most non-stick release coatings, the coating application process just described is efficient, effective, and economical. The pan holders employed in such a process are relatively inexpensive. The pan holding means employed, however, do not hold pans transported thereby tightly in position during the coating application process. A pan blank, which is held in position in the pan holder by its own weight, will shift position during the coating application process. For many applications, however, this pan movement does not seriously adversely affect the quality of the finished product.
Recently, a method has been developed for producing a pattern in the non-stick release coating applied to the interior surface of an article of cookware by magnetically reorienting reflective magnetizable flakes dispersed in the non-stick coating material. Reflective flakes made of a magnetizable material, such as iron, nickel, and alloys containing these metals, such as stainless steel, are mixed in a liquid fluoropolymer non-stick coating material. The non-stick coating material, with the metal flakes mixed therein, is applied to the inside surface of an article of cookware in a conventional manner, e.g., by spraying the material onto the inner surface of the cookware. In the absence of a magnetic field, the magnetizable reflective flakes will be oriented parallel to the cookware surface, i.e., horizontally. However, if a magnetic field is applied to the cookware, either during application of the coating material to the cookware or after application of the coating material but before the coating material dries or hardens, the reflective magnetizable flakes in the magnetic field will become aligned with the magnetic field, i.e., will be reoriented vertically or perpendicular to the cookware surface. When the non-stick coating material is cured, the reoriented magnetizable reflective flakes will be fixed in this vertical position. The horizontally oriented reflective flakes will reflective light back to an observer, the vertically oriented flakes will not. Thus, a pattern, which appears three dimensional to the eye, is created in the non-stick release coating of the cookware. As long as the magnetizable reflective flakes are selected to have the proper dimensions, i.e., no longer than slightly more than the thickness of the fluoropolymer non-stick release coating, the pattern formed in the coating in this manner will not adversely affect the performance of the non-stick release coating.
The shape of the pattern produced in the release coating of the cookware is defined by the shape of the magnetic field applied to the cookware while the fluoropolymer release coating material containing the magnetizable flakes is applied thereto in liquid form. A magnetic field of the desired shape can be achieved by placing a die pattern, e.g., a metal plate in the shape of the desired pattern to be produced in the cookware release coating, against an outside of the cookware, with a magnet (either a permanent magnet or an electromagnetic) placed against the die pattern. The magnet and die pattern in combination produce a magnetic field in the shape of the die pattern on the inside surface of the cookware while the fluoropolymer release coating material containing the magnetizable reflective flakes is applied to the interior surface of the cookware, or after the coating is applied, but before the coating solidifies to thereby reorient a portion of the metal flakes in the coating material in the shape of the die pattern. Such a process is described, for example, in Great Britain Patent No. 1,131,038. This process can, however, produce a "fuzzy" pattern, i.e., a pattern which lacks clarity, in the cookware release coating. When a shaped die pattern is laid directly across the top of the magnet, the resultant imprint of the die pattern formed in the release coating is especially fuzzy where the magnetic force of the magnet is directed through the bulk area of the die pattern. The fuzzy image is a manifestation of unwanted field lines (magnetic background effects). If a stronger magnet is used to try to eliminate the fuzziness of the image, i.e., sharpen the image, another unwanted background effect occurs, namely reproduction of the shape of the magnet itself in the pattern formed in the coating. It has been found that a sharper image pattern may be formed in the release coating if the magnetic force applied to the magnetizable reflective flakes is from a diffused magnetic field, rather than directly from the magnet itself. This is achieved by spacing the magnet, which is the source of the magnetic force, away from the substrate being coated. Magnetic force is thus preferably communicated across the space between the magnet and the flakes in the coating from a diffuse magnetic field intervening between the magnet and the coating through the die pattern of magnetizable material positioned between the diffused magnetic field and the coating on the substrate. The diffused magnetic field isolates the coating from direct exposure to the magnetic field of the magnet, eliminating unwanted background effects from the pattern, thereby improving pattern clarity. A diffused magnetic field may be created by placing a magnetizable diffusion plate between the magnet and the die pattern and/or by spacing the magnet from the dye pattern such that the coating on the substrate is not directly exposed to the magnetic force of the magnet. Such a process is described in U.S. patent application Ser. No. 09/144,766, filed on Sep. 1, 1998 now U.S. Pat. No. 6,103,361.
It is desirable that conventional industrial processes for applying coating materials to cookware articles be adapted to the production of cookware articles having patterns formed in the non-stick release coatings thereof by the magnetic reorientation of reflective magnetizable flakes in the coating material, in the manner described.